This invention relates to the purification of natural gas. More specifically, this invention relates to a method and system for removing sulfur compounds and water present in natural gas.
Raw natural gas must be treated prior to its liquefaction for several reasons. These include removing compounds which interfere with the liquefaction process, with the separation and recovery of hydrocarbon liquids and with meeting the specifications set for the recovered products. For example, the gas must be dried to prevent ice formation during cryogenic operations. Hydrogen sulfide ordinarily must be removed because of its toxic nature. A large number of commercial processes are in use for treating and separating of raw wellhead gas. The steps used in these different processes are each well known to those skilled in the art.
In addition, some natural gas contains mercury at levels as high as 200 to 300 micrograms per cubic meter. For example, the mercury level of natural gas produced from one field is reported in the literature to range from 200 to 330 micrograms per cubic meter. In another field the concentration was reported to range between 15 and 450 micrograms per cubic meter.
The processing of natural gas in LNG plants requires, at some location in the system, contact with equipment made primarily of aluminum. This is particularly true in the stage of processing where the gas has been treated by caustic or carbonate washing to remove CO.sub.2 and H.sub.2 S and then to treatment with liquid amine to complete H.sub.2 S removal. One of the next steps is to chill or cool the gas in aluminum-constructed heat exchangers. Because large volumes of gas must be flowed through the aluminum heat exchangers, they are of a massive size and can represent a capital investment of several million dollars. Damage to these exchangers is to be avoided, if at all possible. One threat of damage comes from the mercury present in the gas flowing through the heat exchangers. Although the concentration of mercury appears low, its effect is cumulative as it amalgamates with the aluminum. The result is damage to the system, such as corrosion and stress cracking, which can lead to equipment failure, fires, and similar catastrophe. Repair of the aluminum heat exchangers damaged by mercury is almost impossible. Replacement of the heat exchangers represents a large expenditure. The down-time results in loss of product production. The problem of mercury in natural gas is discussed further in U.S. Pat. No. 4,094,777 and French Patent 2,310,795, both of which are incorporated herein by reference.
One method for removing mercury utilizes the addition of sulfurous materials, e.g., H.sub.2 S, to precipitate sulfides of mercury from the gas stream. U.S. Pat. No. 4,044,098 to Miller et al., incorporated herein by reference, teaches such a method which further utilizes treatment with organic base (amine) to absorb excess hydrogen sulfide to produce a gas stream of minimal sulfur content with a reduced mercury content. The product of such amine treatment can be passed through a dehydrator which comprises a zeolitic bed. One of the next steps is to chill or cool the gas in heat exchangers and to other additional equipment needed for further processing of the gas.
The zeolitic bed employed in the dehydrator can be damaged by carry-over of the organic base used in H.sub.2 S removal. Organic bases, e.g., alkanolamines, can deposit carbon and destroy the structure and crystallinity of the molecular sieve and render it useless for drying. The resulting shortened life of the zeolitic bed requires unacceptably frequent replacement of the zeolitic bed as well as increased down time for the natural gas processing unit. Accordingly, it would be desirable to extend the service life of the zeolitic bed employed in the dehydrator by reducing carry-over of the organic base to the zeolitic bed.